This invention relates to a pigment-based ink set for ink jet printing that exhibits minimal, if any, chromatic gloss when printed on an image-recording element.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either pigment-based or dye-based. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. A dye is a colorant that is dissolved in the carrier medium. In either case, the carrier medium can be a liquid or a solid at room temperature. Commonly used carrier media are aqueous-based and include water, water miscible humectants and water miscible organic co-solvents.
Pigment-based inks are often preferred over dye-based inks because they render printed images having higher optical densities and better resistance to light and ozone as compared to printed images made with dye-based inks. However, pigment-based inks have their own set of deficiencies that tends to arise from the propensity of the pigment particles to accumulate at the surface of the image-recording element. For example, the pigment particles may exhibit poor rub resistance or cause variations in gloss between imaged areas of different colors in a printed image. Solutions to these problems have been discussed extensively in the patent literature, see for example U.S. Pat. Nos. 6,161,929 and 5,925,178; Eur. Pat. Appl. No. EP 1 108 760 A1; and U.S. patent application Ser. Nos. 10/032,931; 10/033,229; 10/034,281 and 10/034,285 filed on Dec. 28, 2001, the disclosures of which are incorporated herein by reference.
Another deficiency with pigment-based inks is that they may give rise to chromatic gloss in certain imaged areas in a printed image. Chromatic gloss is an artifact that appears as a colored sheen or gloss from the surface of a printed image as the view and/or illumination angle changes relative to the surface of the printed image. In printed images prepared using ink jet inks and image-recording elements, chromatic gloss typically appears as a pink sheen in cyan-colored imaged areas, a yellow-orange sheen in magenta-colored imaged areas and a blue sheen in yellow-colored imaged areas. Chromatic gloss is especially problematic when pigment-based inks are printed on glossy image-recording elements. Although chromatic gloss has been observed for commercially available pigment-based ink sets printed on image-recording elements as will be shown herein, no discussion or acknowledgment of chromatic gloss in ink jet printing has been found in the literature.
Another, similar artifact called bronzing has been described in the field of graphic arts printing, and is sometimes visible for ink jet printed images as well. Although bronzing can appear in samples from the same printer system as chromatic gloss, bronzing appears as a metallic, bronze-colored sheen and is different from chromatic gloss. This invention pertains to chromatic gloss.
Although not designed to minimize chromatic gloss, one possible solution to this problem is to laminate the entire printed image with a protective layer. Another possible solution is to print on an image-recording element having a fusible top layer into which the ink components can penetrate, and then fuse the top layer to generate a smooth glossy surface. Examples of such technology are disclosed in U.S. patent application Ser. No. 09/954,779, filed on Sep. 18, 2001 of Wexler, or coating a protective layer on the imaged areas as described in EP 1 057 646 A1 and EP 1 048 466 A1. However, all these approaches involve separate steps after printing, making the entire process complex and costly.
For the production of high quality photorealistic images via ink jet printing, pigment-based ink sets must provide printed images that are free from objectionable chromatic gloss. It is an object of this invention to provide a pigment-based ink set for ink jet printing that, when printed on an image-recording element, minimizes chromatic gloss to an acceptable level without requiring a separate step after printing.
The present invention overcomes the problems discussed above by providing a pigment-based ink set for ink jet printing, the ink set comprising at least one pigment-based ink, wherein the normalized differential specular reflectance for each ink in the set, Nxcex94RS(X), is less than 1.25 when calculated according to Equation 1:
Nxcex94RS(X)=[Nxcex94RS(Xn)]maxxe2x80x83xe2x80x83Equation 1
where X represents the ink, n is an integer associated with each density patch printed on an image-recording element as part of a test image, and [Nxcex94RS(Xn)]max is the maximum value of the [Nxcex94RS(Xn)] values that are calculated for the density patches printed using ink X; wherein the image-recording element has a gloss value of at least about 5 when measured at 60xc2x0; wherein the test image for ink X consists of a series of at least ten density patches printed such that the ink laydown of the patches increases incrementally, from no ink laydown to a maximum level generating a density of at least about 1.5; and wherein the normalized differential specular reflectance value for each patch, Nxcex94RS(Xn), is determined according to Equation 2:                               N          ⁢                      xe2x80x83                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢                                    R              S                        ⁡                          (              Xn              )                                      =                              [                                                            R                  Smax                                ⁡                                  (                  Xn                  )                                            -                                                R                  Smin                                ⁡                                  (                  Xn                  )                                                      ]                                              R              Smean                        ⁡                          (              Xn              )                                                          Equation        ⁢                  xe2x80x83                ⁢        2            
where RSmax(Xn) is the maximum specular reflectance over the visible spectral region between 390 nm and 720 nm for patch Xn, RSmin(Xn) is the minimum specular reflectance over the visible spectral region between 390 nm and 720 nm for patch Xn, and RSmean(Xn) is the average specular reflectance over the visible spectral region between 390 nm and 720 nm for patch Xn.
As used herein, the term xe2x80x9cink setxe2x80x9d describes all the ink jet inks used in a printer to produce an image. The set can include inks of many colors; inks of many colors with different densities or hues; inks of one color only; or inks of one color with different densities or hues. The term means at least one ink, usually more than one ink, used to produce a color or a black and white image.